t9604: Fix test for musl libc and new Debian

t9604: Fix test for musl libc and new Debian

CST6CDT and the like are POSIX timezone, with no rule for transition.
And POSIX doesn't enforce how to interpret the rule if it's omitted.
Some libc (e.g. glibc) resorted back to IANA (formerly Olson) db rules
for those timezones.  Some libc (e.g. FreeBSD) uses a fixed rule.
Other libc (e.g. musl) interpret that as no transition at all [1].

In addition, distributions (notoriously Debian-derived, which uses IANA
db for CST6CDT and the like) started to split "legacy" timezones
like CST6CDT, EST5EDT into `tzdata-legacy', which will not be installed
by default [2].

In those cases, t9604 will run into failure.

Let's switch to POSIX timezone with rules to change timezone.

1: http://mm.icann.org/pipermail/tz/2024-March/058751.html
2: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1043250

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>